John B. Brady

Metasomatic zones in metamorphic rocks

Abstract Prediction of a unique sequence of metasomatic zones that would develop by intergranular diffusion with local equilibrium is possible only for relatively simple systems, unless extensive thermochemical and kinetic information is available. The complexity of the problem for a given example will depend on what portion of the set of chemical components required to describe the example are ‘diffusing components’, that is, components that move relative to ‘inert markers’. Diffusing components are commonly K-components (Thompson, 1970) for the various local equilibria of a sequence of metasomatic zones, since diffusion tends to impose a monotonie variation of the chemical potentials of these components across the zones. The number of diffusing components may vary from zone to zone in a particular example, as may the number of diffusing components that are K-components. Calculation of the rate of growth of a specific sequence of zones is relatively straightforward only for cases where the zones are primarily due to the variation of the chemical potential of one independent diffusing component. Calculation of the material transfer involved in the growth of a sequence of zones, assuming a single sharp initial contact is meaningful only if ‘inert markers’ or a discontinuity in the otherwise-constant ratio of two components indicate the present location of the initial contact. Examination of some natural calc-silicate diffusion zones suggests that a diffusion-imposed gradient in the chemical potential of calcium is largely responsible for the observed zonation. Metasomatic zones developed at the boundaries of ultramafic bodies, however, are produced by diffusion-imposed chemical potential gradients of several components, notably silica and magnesia, the number varying from zone to zone.

Aragonite pseudomorphs in high-pressure marbles of Syros, Greece

Numerous rod-shaped calcite crystals occur in the blueschist to eclogite facies marbles of Syros, Greece. The rods show a shape-preferred orientation, and the long axes of the rods are oriented at a large angle to foliation. The crystals also have a crystallographic-preferred orientation: calcite c-axes are oriented parallel to the long axes of the rods. Based on their chemical composition, shape, and occurrence in high-pressure marbles, these calcite crystals are interpreted as topotactic pseudomorphs after aragonite that developed a crystallographic-preferred orientation during peak metamorphism. This interpretation is consistent with deformation of aragonite by dislocation creep, which has been observed in laboratory experiments but has not been previously reported on the basis of field evidence. Subsequent to the high-pressure deformation of the aragonite marbles, the aragonite recrystallized statically into coarse rod-shaped crystals, maintaining the crystallographic orientation developed during deformation. During later exhumation, aragonite reverted to calcite, and the marbles experienced little further deformation, at least in the pseudomorph-rich layers. Some shearing of pseudomorph-bearing marble layers did occur and is indicated by twinning of calcite and by a variable inclination of the pseudomorphs relative to foliation.

Evidence for Quaternary Movement on the McKinley Strand of the Denali Fault in the Delta River Area, Alaska

Offset Holocene alluvial fans and drainages along the McKinley strand of the Denali fault near the Delta River in the east-central Alaska Range indicate as much as 50 to 60 m of right-lateral displacement during the last 10,000 yrs. Vertical movement of 6 to 10 m during the same time interval is reflected by south-facing scarps along the trace of the fault. All but possibly 1 m of the lateral movement is thought to predate the 1830 neoglacial ice advance. Older drainages have been offset in a right-lateral sense since early Wisconsin or Illinoian time by as much as 6.5 km or, alternatively, by as little as 1 km.

Advances in the geology of the Tobacco Root Mountains, Montana, and their implications for the history of the northern Wyoming province

Integrated studies by Keck Geology Consortium participants have generated many new insights into the Precambrian geology of the Tobacco Root Mountains. We have clarifi ed the tectonic setting and origin of two suites of metamorphic rocks: (1) a quartzofeldspathic gneiss complex with associated metasupracrustal rocks (the combined Indian Creek and Pony–Middle Mountain Metamorphic Suites) that originated in a continental arc setting between 3.35 and 3.2 Ga with subsequent sedimentation and (2) mafi c metavolcanic rocks with intercalated metasedimentary rocks (the Spuhler Peak Metamorphic Suite) from a suprasubduction zone ophiolite or backarc basin possibly of Proterozoic age. A poorly preserved metamorphic event at 2.45 Ga affected the former but not the latter, as did the intrusion of rift-related mafi c dikes and sills at 2.06 Ga. Both suites were amalgamated, metamorphosed to at least upper amphibolite facies, subjected to simple shear strain and folded into mapand outcrop-scale sheath folds, and tectonically unroofed during the period 1.78 to 1.71 Ga. We name this event the Big Sky orogeny. The Proterozoic geology of the Tobacco Root Mountains can be integrated with coeval features of the geology of the northern Wyoming province to outline a northeast-trending, southeast-vergent belt as the Big Sky orogen. The Big Sky orogen consists of a metamorphic hinterland fl anked to the southeast by a foreland of discrete ductile shear zones cutting older basement, and to the northwest by arc-related metaplutonic bodies and the trace of a fossil subduction zone in the upper mantle. Archean blocks to the north of the Big Sky orogen may have been accreted as allochthonous terranes during collision and convergence. The remarkable synchroneity of collision along the Big Sky orogen with tectonism in the Trans-Hudson orogen along the eastern margin of the Wyoming province and in the Cheyenne belt to the south of the province raise profound but unanswered questions about the process by which the Wyoming province was added to the rest of the ancestral North American craton.

Paleoproterozoic Metamorphism in the Northern Wyoming Province: Implications for the Assembly of Laurentia

U‐Pb ages measured on zircons from the Tobacco Root Mountains and monazite from the Highland Mountains indicate that the northwestern Wyoming province experienced an episode of high‐grade metamorphism at ∼1.77 Ga. Leucosome emplaced in Archean gneisses from the Tobacco Root Mountains contains a distinctive population of zircons with an age of 1.77 Ga but also contains zircons to ∼3.5 Ga; it is interpreted to have been derived primarily by anatexis of nearby Archean schist. A granulite facies mafic dike that cuts across Archean gneissic banding in the Tobacco Root Mountains contains two distinct populations of zircons. A group of small (<50 μm) nonprismatic grains is interpreted to be metamorphic and yields an age of 1.76 Ga; a group of slightly larger prismatic grains yields an age of 2.06 Ga, which is interpreted to be the time of crystallization of the dike. Monazite from a leucogranite from the Highland Mountains yields a well‐defined age of 1.77 Ga, which is interpreted as the time of partial melting and emplacement of the leucogranite. These results suggest that the northwestern Wyoming province, which largely lies within the western part of the Great Falls tectonic zone, experienced a metamorphic maximum at 1.77 Ga. This age is ∼100 m.yr. younger than the proposed time of Wyoming‐Hearne collision in the central Great Falls tectonic zone (1.86 Ga) and suggests that the northwestern Wyoming province may have been involved in a separate, younger collisional event at ∼1.77 Ga. An event at this time is essentially coeval with collisions proposed for the eastern and southeastern margins of the province and suggests a multiepisodic model for the incorporation of the Wyoming craton into Laurentia.

Proterozoic Metamorphism of the Tobacco Root Mountains, Montana

Textures and mineral assemblages of metamorphic rocks of the Tobacco Root Mountains are consistent with metamorphism of all rocks during the Big Sky orogeny (1.77 Ga) at relatively high pressure (P >1.0 GPa) followed by differential Cheney, J.T., Brady, J.B., Tierney, K.A., DeGraff, K.A., Mohlman, H.K., Frisch, J.D., Hatch, C.E., Steiner, M.L., Carmichael, S.K., Fisher, R.G.M., Tuit, C.B., Steffen, K.J., Cady, P., Lowell, J., Archuleta, L.L., Hirst, J., Wegmann, K.W., and Monteleone, B., 2004, Proterozoic metamorphism of the Tobacco Root Mountains, Montana, in Brady, J.B., Burger, H.R., Cheney, J.T., and Harms, T.A., eds., Precambrian geology of the Tobacco Root Mountains, Montana: Boulder, Colorado, Geological Society of America Special Paper 377, p. 105–129. For permission to copy, contact editing@geosociety.org.

Precambrian meta-ultramafic rocks from the Tobacco Root Mountains, Montana

Meta-ultramafi c rocks occur as small (2 to 100 m long), podiform bodies in all three major Precambrian rock suites of the Tobacco Root Mountains of southwest Montana. Most samples consist of a randomly oriented, coarse-grained assemblage of orthopyroxene, olivine, and magnesiohornblende ± spinel, partially replaced by a fi ne-grained assemblage that may include anthophyllite, talc, cummingtonite, magnesiohornblende, chlorite, serpentine, and/ or magnetite. Blackwall reaction zones of anthophyllite, actinolite, chlorite, and/or biotite surround several of the meta-ultramafi c bodies. The absence of clinopyroxene with orthopyroxene limits the metamorphic pressure-temperature history of these rocks to temperatures below ~800 °C. The presence of anthophyllite with olivine indicates that these rocks passed through 650‐700 °C at pressures below 0.6 GPa. These constraints are consistent with the detailed pressure-temperature path determined for the surrounding upper-amphibolite to granulite facies metamorphic rocks. Whole-rock chemical analyses of the meta-ultramafi c rocks show them to be rich in SiO 2 (44‐54 wt%) and poor in MgO (21‐34 wt%) relative to mantle peridotites and typical Alpine-type ultramafi c rocks. Rare earth element concentrations are all enriched relative to chondritic values and the light rare earth elements are especially enriched (10‐30 times), inconsistent with either an upper-mantle or a ko matiitic origin. All samples have similar TiO 2 /Zr ratios, suggesting that they have a common or related origin, and that TiO 2 and Zr were conserved in the processes that led to the observed chemical variations. Together, the chemical data point to a protolith that was an ultramafi c cumulate rich in orthopyroxene, and therefore probably formed in a continental setting from a basaltic magma enriched in silica. One possible time of origin is a magmatic event during the continental rifting at 2.06 Ga that led to the intrusion of a suite of mafi c dikes.

General geology and geochemistry of metamorphosed Proterozoic mafic dikes and sills, Tobacco Root Mountains, Montana

Just over two billion years ago, basaltic magma intruded rocks of the Pony– Middle Mountain Metamorphic Suite and the Indian Creek Metamorphic Suite that now crop out in the Tobacco Root Mountains of Montana near the northwestern margin of the Wyoming province. Numerous examples can be found of mafi c dikes that crosscut layering and gneissic banding, demonstrating that the host rocks were metamorphosed to form the gneissic texture prior to intrusion of the dikes. Although many of the intrusions appear to be sills that followed compositional layering, close inspection reveals low-angle discordance in nearly every case, consistent with rotation of dikes by shearing into nearly layer-parallel orientations. The mafi c dikes do not intrude the adjacent Spuhler Peak Metamorphic Suite, which we interpret to mean that the Spuhler Peak Metamorphic Suite was not present at the time of intrusion. These dikes and sills were metamorphosed along with their host rocks during the Big Sky orogeny, a major orogenic event at 1.77 Ga that is documented in this volume. The fi ne-grained, garnet-bearing, rusty-weathering metamorphosed mafi c dikes and sills (MMDS) generally have a distinctive appearance in the fi eld. Some MMDS were clearly folded or boudinaged during metamorphism, but many show only weak foliation and still have sharp contacts with their host rocks. A decrease in grain size commonly occurs on the margins of the MMDS and is believed to be a metamorphic texture that developed from the chilled margins of intrusions into cold rocks. Chemically, the MMDS are subalkaline tholeiites that have been modifi ed by signifi cant fractional crystallization, as evidenced by molar Mg/(Mg + Fe) values that vary from 0.6 to 0.3 and TiO 2 values that vary from 0.6 to 3.0 wt%. The ratio of TiO 2 to P 2 O 5 is virtually constant, indicating a common origin for the MMDS and that these two components were similarly incompatible with the fractionating minerals. The weight Brady, J.B., Mohlman, H.K., Harris, C., Carmichael, S.K., Jacob, L.K., and Chaparro, W.R., 2004, General geology and geochemistry of metamorphosed Proterozoic mafi c dikes and sills, Tobacco Root Mountains, Montana, in Brady, J.B., Burger, H.R., Cheney, J.T., and Harms, T.A., eds., Precambrian geology of the Tobacco Root Mountains, Montana: Boulder, Colorado, Geological Society of America Special Paper 377, p. 89–104. For permission to copy, contact editing@geosociety.org.

40Ar/39Ar ages of metamorphic rocks from the Tobacco Root Mountains region, Montana

Measurements of 60 single-grain, UV laser microprobe 40 Ar/ 39 Ar total gas ages for hornblende from metamorphic rocks of the Tobacco Root Mountains in southwest Montana yield a mean age of 1.71 ± 0.02 Ga. Measurements of 40 Ar/ 39 Ar step-heating plateau ages of three bulk hornblende samples from the Tobacco Root Mountains metamorphic rocks average 1.70 ± 0.02 Ga. We believe that these and the K/Ar or 40 Ar/ 39 Ar ages reported by previous workers are cooling ages from a 1.78 to 1.72 Ga, upper-amphibolite to granulite facies, regional metamorphism (Big Sky orogeny) that affected the northwestern portion of the Wyoming province, including the Tobacco Root Mountains and adjacent ranges. Based on the 40 Ar/ 39 Ar data, this 1.78–1.72 Ga metamorphism must have achieved temperatures greater than ~500 °C to reset the hornblende 40 Ar/ 39 Ar ages of samples from the Indian Creek Metamorphic Suite, which was previously metamorphosed at 2.45 Ga, and of the crosscutting metamorphosed mafi c dikes and sills (MMDS), which were intruded at 2.06 Ga. Biotite and hornblende from the Tobacco Root Mountains appear to give the same 40 Ar/ 39 Ar or K/Ar age (within uncertainty), indicating that the rocks cooled rapidly through the interval from 500 to 300 °C. This is consistent with a model of the Big Sky orogeny that includes late-stage tectonic denudation that leads to decompression and rapid cooling. A similar cooling history is suggested by our data for the Ruby Range. Three biotite samples from the Ruby Range yield 40 Ar/ 39 Ar step-heating plateau ages with a mean of 1.73 ± 0.02 Ga, identical to the best-estimate (near-plateau) age for a hornblende from the same rocks. Two samples of the orthoamphibole, gedrite, from the Tobacco Root Mountains were studied, but did not have enough K to yield a reliable 40 Ar/ 39 Ar age. Several biotite and three hornblende samples from the region yield 40 Ar/ 39 Ar dates signifi cantly younger than 1.7 Ga. We believe these samples were partially reset during contact metamorphism by Cretaceous (75 Ma) intrusive rocks. Hydrothermal alteration associated with ca. 1.4 Ga rifting led to growth of muscovite with that age in the Ruby Range, but this alteration was apparently not hot enough to reset biotite and hornblende ages there.

Does Ice Dissolve or Does Halite Melt? A Low-Temperature Liquidus Experiment for Petrology Classes.

Measurement of the compositions and temperatures of H2O-NaCl brines in equilibrium with ice can be used as an easy, in-class, experimental determination of a liquidus. The experiment emphasizes the symmetry of the behavior of brines with respect to the minerals ice and halite (if hydrohalite is ignored) and helps free students from the conceptual tethers of one-component ice melting. Furthermore, examining a familiar, low-temperature chemical system using the diagrams and nomenclature of igneous petrology builds a sturdy bridge toward the understanding of igneous processes.